This invention relates to an apparatus for measuring properties of a material flow through apparatus such as a conduit of a Coriolis mass flowmeter. More particularly, this invention relates to calibrating a driver affixed to the conduit to excite the conduits in only a desired mode of vibration. Still more particularly, this invention relates to determining a drive signal that causes the driver to vibrate the conduit in a desired mode of vibration.
It is known to use Coriolis effect mass flowmeters to measure mass flow and other information for materials flowing through a conduit in the flowmeter. Exemplary Coriolis flowmeters are disclosed in U.S. Pat. No. 4,109,524 of Aug. 29, 1978, U.S. Pat. No. 4,491,025 of Jan. 1, 1985, and U.S. Pat. No. Re. 31,450 of Feb. 11, 1982, all to J. E. Smith et al. These flowmeters have one or more conduits of a straight or a curved configuration. Each conduit configuration in a Coriolis mass flowmeter has a set of natural modes of vibration, which may be of a simple bending, torsional or coupled type. Each conduit is driven to oscillate at a resonance in one of these natural modes of vibration. Material flows into the flowmeter from a connected pipeline on the inlet side of the flowmeter, is directed through the conduit or conduits, and exits the flowmeter through the outlet side of the flowmeter. The natural vibration modes of the vibrating, material filled system are defined in part by the combined mass of the conduits and the material flowing within the conduits.
When there is no flow through the flowmeter, all points along the conduit oscillate, due to an applied driver force, with identical phase or a small initial fixed phase offset which can be corrected. As material begins to flow, Coriolis forces cause each point along the vibrating conduit to have a different phase. The phase on the inlet side of the conduit lags the driver, while the phase on the outlet side of the conduit leads the driver. Pick-offs are placed on the conduit(s) to produce sinusoidal signals representative of the motion of the conduit(s). Signals output from the pick-offs are processed to determine the phase difference between the signals. The phase difference between two pick-offs signals is proportional to the mass flow rate of material through the conduit(s).
An essential component of every Coriolis flowmeter and of every vibrating tube densitometer is the drive or excitation system. The drive system operates to apply a periodic physical force to the conduit which causes the conduit to oscillate. The drive system includes a driver mechanism mounted to the conduit(s) and a drive circuit for generating a drive signal to operate the drive mechanism. The driver mechanism typically contains one of many well known arrangements, such as a magnet mounted to one conduit and a wire coil mounted to the other conduit in an opposing relationship to the magnet.
The drive circuit continuously applies a periodic drive signal to the drive mechanism. The drive signal is typically sinusoidally or square shaped. In a typical magnetic-coil drive mechanism, the periodic drive signal causes the coil to produce an alternating magnetic field. The alternating magnetic field of the coil and the constant magnetic field produced by the magnet force causes the flow conduit(s) to vibrate in a sinusoidal pattern. Those skilled in the art recognize that any device capable of converting an electrical signal to mechanical force is suitable for application as a driver. (See, U.S. Pat. No. 4,777,833 issued to Carpenter and assigned on its face to Micro Motion, Inc.) Also, one need not use a sinusoidal signal. Any periodic signal may be appropriate as the driver signal (See, U.S. Pat. No. 5,009,109 issued to Kalotay et. al. and assigned on its face to Micro Motion, Inc.).
For a dual tube flowmeter, a typical mode, although not the only mode, in which Coriolis flowmeters are typically driven to vibrate is a first out-of-phase bending mode. The first out-of-phase bending mode is the fundamental resonant bending mode at which the two conduits of a dual tube Coriolis flowmeter vibrate in phase opposition. However, this is not the only mode of vibration present in the vibrating structure of the Coriolis. Higher modes of vibration may be also be excited in the conduits. For example, a first out-of-phase twist mode may be excited as a result of material flowing through the vibrating conduit and the consequent Coriolis forces caused by the flowing material. Other higher modes of vibration that may be excited include in-phase bending and lateral modes of vibration.
Hundreds of vibration modes may be excited in a Coriolis flowmeter that is driven in the first out-of-phase bending mode. Even within a relatively narrow range of frequencies near the first out-of-phase bending mode, there are at least several additional modes of vibration that may be excited by the drive system. In addition to multiple modes being excited by the driver, undesired modes of vibration can also be excited due to vibrations external to the flowmeter. For example, nearby machinery in a process line might generate a vibration that excites an unwanted mode of vibration in a Coriolis flowmeter.
The drive system can excite additional and undesirable modes of vibrations because the driver mechanism is not ideal. The drive system is comprised of the drive circuitry generating a command signal and a driver mechanism that converts the received command signal into a force. An ideal driver mechanism would be linear, and the mechanism would generate a force that is linearly related to the command signal. However, the relationship between the command signal applied to the driver and the force generated by the driver is non-linear due to various reasons. Manufacturing tolerances require that driver elements be located symmetrically on the conduits. Any resulting non-linearity causes a distortion in the drive force, which can appear as forces applied to the structure at harmonics of the original drive signal. The drive system is configured to apply a drive signal to the driver that applies a sufficient force to the conduit(s) to vibrate them in the desired mode of vibration. However, when the driver is not ideal, the force applied to the conduit(s) is not ideal, and forces at higher frequencies are generated. These higher frequency forces may excite other unwanted structural modes.
The application of eccentric forces excites multiple modes of vibration in the conduits. Thus, a Coriolis flowmeter driven to oscillate or resonate in a desired mode of vibration, such as the first out-of-phase bending mode, may actually have a conduit(s) oscillating in many other modes in addition to the desired mode. Meters driven to oscillate in a different mode other than the first out-of-phase bending mode experience the same phenomenon of having multiple excited modes of vibration in addition to the intended drive mode.
The application of eccentric forces by a conduit on a driver can be a particular problem if an apparatus, such as a Coriolis flowmeter, is unbalanced. An apparatus is balanced when vibrations within the apparatus cancel one another out to create a zero sum vibration of the apparatus. Apparatus is unbalanced when vibration are not canceled out. This causes a force to be added to a system. A typical dual conduit apparatus, such as a dual tube Coriolis flowmeter, is balanced because the two conduits vibrate in phase opposition to one another and cancel opposing vibration. However, unbalanced apparatus does not have a conduits vibrating in opposite directions to cancel the vibrational forces from the conduit.
Unbalance can cause significant coupling between the ambient environment and the conduit(s). This coupling increases the impact of structural dynamics of the ambient environment, and may cause an undesired mode of vibration to be excited by a harmonic of the force applied by the driver to the conduit. Therefore, it is desirable in an unbalance apparatus to have a driver applying a force that only excites a desired mode of vibration.
For the above reasons, there is a need for a drive circuit system for vibrating conduits in an apparatus for measuring that drives the vibrating tube(s) of the flowmeter reduces the undesired modes of vibration that are excited by a driver oscillating the conduits.
The above and other problems are solved and an advance in the art is achieved by the provision of a system for calibrating a drive signal. The calibration system of the present invention determines the proper drive signal to apply to a driver affixed to a conduit for measuring properties of a material flowing through the conduit. This enables the driver to apply a force that vibrates the conduit in a desired mode of vibration. By determining the proper the drive signal, harmonic forces applied to a conduit by a driver are reduced. This increases the amount of vibration in the desired mode of vibration and reduces the amount of vibrations in undesired modes of vibration. The decrease of vibrations in undesired modes of vibration greatly decreases noise floor to which the flowmeter is subjected. The amplification of the desired response and the decrease in noise allow more accurate measurements of properties, such as mass flow rate to be made. Furthermore, repeatability of measurements is improved.
In order to calibrate the proper drive signal, a pick-off, such as an accelerometer, is affixed to a conduit proximate a driver. A computer or digital processor excites the driver with a broad band noise signal. This causes the driver to apply a band limited noise force to the conduit and the pick-off to measure the motion of the vibrating structure at the driver. The digital processor then receives data about the motion of the structure from the pick-offs. The data is used by the digital processor to generate a dynamic model of the structure and driver. The digital processor uses the dynamic model to determine a drive signal that will cause the driver to apply a force to the conduit which will vibrate the conduit in a desired mode of vibration.
In one embodiment, the digital processor outputs the result to a display or the like and a conventional analog drive circuit connected to the driver is configured to generate the proper drive signal. In the preferred embodiment, the conventional drive circuit can be configured by setting a reference voltage in the drive circuit.
In an alternative embodiment, meter electronics contain a digital signal processor which controls the drive signal applied to the driver. When a digital signal processor is used in the meter electronics, the calibration can be performed periodically by the digital signal processor to adjust the drive signals as the structural dynamic of the apparatus changes. This is done by periodically vibrating the conduits with the driver. The data from vibration is then stored in a memory. The stored data is then used with the data from the current vibration to detect the new structural dynamic of the apparatus and then determine a new drive signal.
An aspect of the invention is a method for calibrating a drive signal to be applied to a driver to cause said driver to oscillate at least one flow tube in a flow measuring apparatus for measuring properties of a material flow in said at least one flow tube, the method comprising the steps of:
vibrating said at least one flow tube with said driver;
measuring vibrations of said at least one flow tube responsive to vibrating said flow tube; and being characterized by the steps of:
detecting physical characteristics of said apparatus and said driver from said measured vibrations of said at least one flow tube; defining a drive signal for said driver in response to the detection of said physical characteristics, and
applying said defined drive signal to said driver to oscillate said at least one flow tube in a desired mode of vibration.
Another aspect is a method wherein said flow measuring apparatus is a flowmeter and the method is for calibrating a drive signal to be applied to a driver of said flowmeter, said method being characterized by the steps of:
applying a first signal to said driver to vibrate said at least one flow tube;
measuring vibrations of said at least one flow tube and said driver responsive to vibrating said at least one flow tube when said first signal is applied to said driver;
determining physical vibrational characteristics including any undesired physical vibrational characteristics of said flowmeter including said flow tube and said driver in response to said measurement of said vibrations of said at least one flow tube when said first signal is applied to said driver;
determining a correction factor for said determined physical vibrational characteristics;
utilizing said determined physical vibrational characteristics and said correction factor to define a drive signal to be applied to said driver; and
applying said defined drive signal to said driver to oscillate said at least one flow tube in a desired mode of vibration that compensates for said undesired physical vibrational characteristics;
Another aspect is the step of storing a measurement of said vibration of said at least one flow tube where said step of detecting said physical characteristics of said apparatus further comprises the step of:
combining a current measurement of vibrations and said stored measurements of vibrations to determine said physical characteristics.
Another aspect comprises the step of setting a reference voltage in a drive signal circuit responsive to determining said drive signal to cause said drive signal circuit to generate said drive signal.
Another aspect further comprising the step of periodically repeating said method for calibrating said drive signal.
Another aspect is wherein said step of detecting said physical characteristics of said driver and apparatus comprises the step of modeling dynamics of said apparatus and driver from said measured vibrations.
Another aspect further comprising the step of modifying drive circuitry to generate said drive signal.
Another aspect is said step of determining said drive signal comprises the steps of:
calculating coefficients of polynomials for a function relating a drive signal to force applied to said conduits;
determining coefficients of polynomials of an inverse function of said function for relating said drive signal to said force; and
inserting a command level of said drive signal into said inverse function to determine said drive signal.
Another aspect comprises meter electronics for an apparatus for measuring properties of a material having at least one flow tube that receives material, a driver for vibrating said at least one flow tube, pick-off sensors for measuring vibrations of said at least one conduit at points along said flow tube, said meter electronics including drive circuitry for applying a drive signal to said driver to cause said drive to oscillate at least flow tube and circuitry for measuring properties of a material flow said at least one flow tube, said meter electronics further comprising the steps of:
first circuitry that receives signals from said pick-off sensors and detects physical characteristics of said apparatus and said driver from said measured vibrations of said at least one flow tube; and
second circuitry that determines a drive signal that causes said driver to oscillate said at least one flow tube in a desired mode of vibration from said physical characteristics of said apparatus and said driver.
Another aspect further comprises a memory that stores measurements of vibrations of said at least one flow tube.
Another aspect is that said first circuitry further comprises circuitry that combines a current measurement of vibrations and said stored measurements of vibrations to determine said physical characteristics.
Another aspect comprises a reference voltage in a drive signal circuit set to a level to generate said drive signal responsive to determining said drive signal to cause said drive signal circuit to generate said drive signal.
Another aspect comprises timing circuitry that periodically repeats calibration of said drive signal.
Another aspect is that said first circuitry comprises modeling circuitry that models dynamics of said apparatus and driver from said measured vibrations.
Another aspect is that said second circuitry includes:
circuitry that calculates coefficients of polynomials for a function relating a drive signal to force applied to said conduits;
circuitry that determines coefficients of polynomials of an inverse function of said function for relating said drive signal to said force; and
circuitry that inserts a command signal of said current drive signal into said inverse function to determine said drive signal.
Another aspect comprises a product for calibrating a drive signal to apply to a driver to cause said drive to oscillate at least flow tube in an apparatus for measuring properties of a material flowing through said at least one flow tube, said product comprising:
instructions for directing a processor to:
receive signals measuring vibrations of said at least one flow tube from pick-off sensors affixed to said at least one flow tube, and
detect physical characteristics of said apparatus and said driver from said measured vibrations of said at least one flow tube, and
determine a drive signal that causes said driver to oscillate said at least one flow tube in a desired mode of vibration from said physical characteristics of said apparatus and said driver; and
a media readable by said processor that stores said instructions.
Another aspect is that said instructions further comprise instructions directing said processor to apply said drive signal to said driver to cause said driver to oscillate said at least one flow tube in said desired mode.
Another aspect is that said instructions further comprise instructions for directing said processor to store measurements of said vibrations of said at least one flow tube in a memory.
Another aspect is that said instruction for processor to detect said physical characteristics of said apparatus and said driver further comprise:
instructions for directing said processor to read said measurements from said memory and to combine a current measurement of vibrations and said stored measurements of vibrations to determine said physical characteristics.
Another aspect is that said instructions further comprise:
instructions for directing said processor to display a reference voltage for a drive signal circuit responsive to determining said drive signal to cause said drive signal circuit to generate said drive signal.
Another aspect is that said instructions further comprise instructions for directing said processor to periodically repeat calibration said drive signal.
Another aspect is that said instruction for detecting said physical characteristics of said driver and apparatus comprises instructions for directing said processor to model dynamics of said apparatus and driver from said measured vibrations.
Another aspect is that said instructions for directing said processor to determine said drive signal comprises:
instructions for directing said processor to calculate coefficients of polynomials for a function relating a drive signal to force applied to said conduits, determine coefficients of polynomials of an inverse function of said function for relating said drive signal to said force, and insert a command signal of said drive signal into said inverse function to determine said drive signal.
Another aspect comprises a method for calibrating a drive signal for a driver to cause said drive to oscillate at least one flow tube in apparatus for measuring properties of a material flow within said at least one flow tube, said method comprising the steps of:
vibrating said at least one flow tube by said driver;
measuring vibrations of said at least one flow tube responsive to said vibratings;
detecting physical characteristics of said apparatus and said driver from said measured vibrations of said at least one flow tube; and
determining a drive signal that causes said driver to oscillate said at least one flow tube in a desired mode of vibration from said physical characteristics of said at least one flow tube and said driver.